1. Field of the Invention
The present invention relates generally to systems and methods for controlling spacecraft or satellites, and in particular to a system and method for stepping spacecraft mechanisms to minimize disturbances generated by the stepping mechanism.
2. Description of the Related Art
Three-axis stabilized spacecraft or satellites often include mechanisms for manipulating appendages. These mechanisms include mechanisms that rotate the antenna reflectors to specific targets, gimbals that scan a payload image frame across a specific area of the Earth's surface, and solar array drivers that step solar arrays to track the Sun. Typically, such mechanisms use simple, reliable stepper motors coupled to the spacecraft component (payloads or solar arrays) via gear-driven transmissions. Stepper motors are desirable because they are relatively simple to control, reliable, lightweight and well adapted to continuous use. The stepper motors and transmissions are used to rotate the solar array along its longitudinal axis to track the sun while the spacecraft orbits about the Earth. The rate that the solar array must be rotated is a function of the satellite orbital period. At geosynchronous orbit, this rate is about 0.004 degrees per second.
The use of a stepper motor in spacecraft with highly flexible structural components such as large deployable payload booms, antenna reflectors, and solar arrays may potentially excite some structural modes of these components and generate significant oscillation disturbances in the spacecraft itself. This disturbance can degrade the spacecraft pointing, cause excessive activity of the spacecraft control actuators, and make autonomous spacecraft momentum dumping difficult. The induced oscillation is particularly critical in spacecraft where absolute platform stability is desirable. Vibrations can cause deterioration of any inertia-sensitive operations of a spacecraft.
This disturbance problem can be alleviated by a number of techniques. One technique is to employ high bandwidth control loops to mitigate the impact of this disturbance to the spacecraft pointing. This technique, however, has significant limitations. For many spacecraft, the structural modes that are excited by the stepping mechanisms are outside of the spacecraft control bandwidth. Consequently, these high-bandwidth control loops have only very limited effects on the disturbance. Further extension of the bandwidth of the control loops to include these structural modes will very often result in control loop stability problems. Furthermore, high-bandwidth control also unnecessarily increases actuator operation, which can increase wear and result in excess energy consumption.
Another technique for mining the solar array drive stepping disturbance is disclosed in U.S. Pat. No. 4,843,294, entitled “Solar Array Stepping to Minimize Array Excitation,” issued Jun. 27, 1989 to Bhat et al, which is hereby incorporated by reference herein. In this reference, mechanical oscillations of a mechanism containing a stepper motor, such as a solar array powered spacecraft, are reduced and minimized by the execution of step movements in pairs of steps. The period between steps is equal to one-half of the period of torsional oscillation of the mechanism. While this method can reduce structural disturbances, it is not very effective when the mechanism has significant backlash and stiction. This is because the backlash and stiction can significantly interrupt the two-step pattern of this method.
Another technique is described in co-pending and commonly-assigned patent application Ser. No. 10/386,796, entitled “METHOD AND APPARATUS FOR STEPPING SPACECRAFT MECHANISMS AT LOW DISTURBANCE RATES,” by Ketao Liu, filed on Mar. 12, 2003, in which transients due to the interaction between appendage stepping and resonances are reduced by deadbeating at a half resonance cycle between the North and South wings. This technique, however, is subject to frequency sensitivities and uncertainties, and cannot be implemented in all existing spacecraft.
Still another technique is described in co-pending and commonly-assigned patent application Ser. No. 10/348,663, entitled “METHOD AND APPARATUS FOR MINIMIZING SOLAR ARRAY SUN TRACKING DISTURBANCE VIA NORTH AND SOUTH SOLAR ARRAY WING CANCELLATION,” by Ketao Liu, filed Mar. 12, 2003. The technique, however, is a more cumbersome implementation.
There is therefore a need for a system and method for minimizing disturbances in stepper-motor driven mechanisms that are more robust to mechanism backlash and stiction. The present invention satisfies that need.